Cathodic sputtering is widely used for the deposition of thin layers of material onto desired substrates. Basically, this process requires a gas ion bombardment of a sputter target having a face formed of a desired material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target materials to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated beneath or around a backing plate that is positioned in a heat exchange relationship with the target. The sputter target forms a part of a cathode assembly that, together with an anode, is placed in an evacuated chamber filled with an inert gas, preferably argon. An electric field is applied across the cathode and the anode. The inert gas is ionized by collision with electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, these ions dislodge the target material. The dislodged target material traverses the evacuated enclosure and deposits as a thin film on the desired substrate, which is normally located close to the anode.
In a conventional target cathode assembly, the sputter target is attached at a single bonding surface to a nonmagnetic backing plate to form a parallel interface in the assembly. The backing plate provides a means for holding the target assembly in the sputtering chamber and provides structural stability to the target assembly. Also, the backing plate is normally water-cooled to carry away the heat generated by the ion bombardment of the target.
To achieve good thermal and electrical contact between the sputter target and the backing plate, these members are commonly attached to each other by use of soldering, brazing, diffusion bonding, solid state bonding, explosion bonding, mechanical fastening or epoxy bonding. The bond between the sputter target and the backing plate must accommodate stresses exerted on the target/backing plate assembly that occur upon cooling. If the bond is not sufficiently strong, the sputter target and backing plate will debond during service.
Quality assurance testing is performed by sputter target/backing plate assembly manufacturers and users to evaluate the strength of the bond interface to minimize occurrence of debonding during sputtering.
The current method for testing bond interfaces involves machining an evaluation sample to tight dimensions in which the thickness of the target layer is equal to the thickness of the backing plate layer, then clamping the sides of each of the component layers and subjecting the sample to a pull test. By this method, the evaluation samples are difficult to prepare due to the precise machining required, and the test results have proven unpredictable and unreliable. Often, the component materials, which are being squeezed by the clamps, yield before the bond interface is evaluated. There is uncertainty as to whether the results of the test apply to the strength of the bond interface or the strength of the materials comprising the sputter target and backing plate.
There is thus a need for a method of testing the strength of a bond interface in which the evaluation sample is simple to prepare and the results are repeatable and reliable.